Electrochemical fuel cells convert fuel such as H2 or methanol, and oxidant such as air or O2, to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) in which an electrolyte in the form of an ion-exchange membrane is disposed between an anode layer and a cathode layer. These electrode layers are made from porous, electrically conductive sheet material such as carbon fiber paper or carbon cloth. In a typical MEA, the electrode layers provide structural support to the membrane which typically thin and flexible. The MEA contains an electrocatalyst that typically is composed of finely divided platinum particles in a layer at each membrane/electrode layer interface, to catalyze the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
Hydrogen/air polymer electrolyte fuel cells (PEFCs) are considered a promising technology to replace internal combustion engines for automotive propulsion. However, a major drawback of current PEFC technology is their high cost, largely due to the use of platinum-based catalysts at both the anode (10%) and cathode (90%) [1].
Two general paths have been considered to reduce the cost of PEFC cathode catalysts. One path is to improve the activity for oxygen reduction reaction (ORR) of platinum-based catalysts by nano-structuring or alloying. Another path is to replace the platinum-based catalysts with lower cost, non-precious metal catalysts (NPMCs) [2]. A major challenge is developing NPMCs that are both highly active and durable because high activity is often couple with fast degradation [3].
NPMCs have been prepared using nitrogen-containing polymeric precursors including ethylenediamine, polypyrrole, and polyaniline (PANT) [4-6]. Graphene structures were observed from highly magnified images of at least some of these catalysts. It is possible that the graphene present in these materials might provide stability to the catalyst [4-7].
Corrosion of carbon supports likely contributes to the degradation of carbon-supported NPMC-based electrocatalysts [8]. Carbon black supports are used most widely with fuel cell electrocatalysts due to a good balance of electron conductivity, surface area, and cost. Although slow at voltages below approximately 1.2 V, carbon corrosion is thermodynamically favorable at voltages higher than 0.20 V, which are typical conditions for fuel cell cathode operation. Therefore, many carbon black-supported ORR electrocatalysts, in particular precious metal electrocatalysts, suffer from performance loss caused by carbon corrosion [9,10].
Carbon nanotubes have been considered as supports for electrocatalysts in fuel cells due to their high electron conductivity and corrosion resistance [11]. Some studies have shown improved performance of Pt electrocatalysts for methanol oxidation and oxygen reduction reactions using single-walled and multi-walled carbon nanotubes as support materials [12-14].
A need remains for active, durable, non-precious metal electrocatalysts for the oxygen reduction reaction for fuel cells.